1. Field of the Invention
The present invention relates to a method of producing a metal mold easily by using a low melting-point alloy.
2. Description of the Related Art
Conventionally, to produce a metal mold easily, a method has been generally adopted in which a gypsum pattern and a sand mold are fabricated consecutively from a matrix by means of transfer, and a low melting-point alloy (mold material) such as a zinc alloy is cast into the sand mold ("Metal Press" November 1976, pp. 16 and 17). According to this method, however, there is a problem that transfer processes are numerous and a long period is required for fabrication of the mold, resulting in a high cost burden.
Accordingly, a dual forming method has been proposed in which a press apparatus and a melting apparatus are formed integrally, and a forming model (matrix) having a product thickness of such as a sheet metal model and provided with bores is immersed in a melt of a low melting-point alloy in a melting tank and is allowed to solidify as it is, thereby simultaneously obtaining a punch and a die (Japanese Patent Publication No. 7576/1973). As a modification of the dual forming method, a method is also known in which, by using a forming model having no bores, an outer surface of the forming model is brought into contact with a melt of a low melting-point alloy, and an alloy of a type different from the aforementioned alloy is poured into its inner surface and is allowed to solidify as it is, thereby simultaneously obtaining a punch and a die of different types of alloy (Japanese Patent Publication No. 15969/1978, Japanese Patent Laid-Open No. 55733/1976, etc.). These methods have already been established and put to practical use. According to these methods, it is possible to obtain a punch and a die simultaneously via a forming model. The efficiency with which metal molds are produced can be improved substantially over the widely practiced method using the aforementioned sand mold.
However, with these methods using forming models, it is necessary to fabricate the forming models with high accuracy in order to ensure a clearance between the punch and the die. Hence, there have been drawbacks that the fabrication of the forming models is very troublesome, and that it is impossible to readily cope with design changes.
Meanwhile, Japanese Patent Publication No. 6014/1980 discloses a method wherein a heat resistant resin sheet is attached to a matrix such as a wooden pattern, and this assembly is accommodated in a casting frame, into which a melt of a low melting-point alloy is subsequently cast to produce a female die. The resin sheet is transferred onto and attached to the female die by the heat generated at that time, the casting frame is inverted, and the melt of a low melting-point alloy is poured into the female die, thereby casting a male die. According to this method, since it is possible to ensure a die clearance by means of the resin sheet, the aforementioned forming model becomes unnecessary, so that various problems encountered in the use of the forming models can be overcome.
However, since the aforementioned heat resistant resin sheet is an adhesive sheet which thermosets afterwards, if an attempt is made to bring the sheet into close contact with the product configuration surface of the matrix, a local elongation unavoidably occurs, and the occurrence of wrinkles and the trapping of air cannot be avoided. For this reason, so-called patching work must be performed by dividing the sheet into a multiplicity of small pieces. Consequently, much labor and skill are required in the same way as in the attachment of sheet wax, with the result that a decline in the work efficiency becomes unavoidable.